According to the Monster Manual Owlbears are a result of magic experiments gone awry. That would be a reason not to be beasts and different from a Platypus which is a natural animal.There is no particular reason for the owlbear to be a monstrosity rather than a beast any more than there is for the platypus.
Not saying you have to use that fiction in your home campaign, but in terms of the official rules they are not normal animals.